In a battery,a halogen retention vent means

ABSTRACT

ELECTRODE STRUCTURES FOR A REACHARGEABLE METAL HALIDE BATTERY INCLUDE: A CATHODIC ELECTRODE COMPRISING A HALOGEN-INERT ELECTROCONDUCTIVE LAYER AND BONDED TO ONE OF THE MAJOR SURFACES THEREOF, A HALOGEN-ENTRAPMENT STRUCTURE COMPRISING A HALOGEN-ADSORBENT LAYER AND A SURFACE LAYERCOMPRISING POROUS, ELECTRICALLY NON-CONDUCTIVE HALOGEN-INERT, ELECTROLYTE-INERT, AND HALOGEN NON-ADSORBENT PARTICLES,HAVING AN AVERAGE LARGEST DIMENSION LESS THAN   ABOUT 10 MILS AND A HALOGEN-INERT BONDING AGENT BONDING THE PARTICLES TOGETHER INTO AN INTEGRAL NON-CONDUCTIVE MASS WHICH ESSENTIALLY RETAINS THE POROSITY OF THE PARTICLES; AND AN ANODIC ELECTRODE HAVING A COATING OF SUCH ELECTRICALLY NONCONDUCTIVE PARTICLES ON THE ELECTROPLATING SURFACE OF THE ELECTRODE.

R. zlTo, JR 3,827,915 IN A BATTERY, A HALOGEN RETENTION VENT IEANS Aug.6, 1974 'Original Filed Feb. 5, 1971 3 Sheets-Shoot 1 22 95 z EE Aug. 6,1974 R. ZITQ, JR

IN A BATTERY, A HALOGEN RETENTION VENT MEANS Original Filed Feb. 3, 197122 v V 7 29 /,/4e

FIGS

3 Sheets-Sheet 2 /AV L\ FIG 4 R. ZITO, JR

Aug. 6, 1974 IN A BATTERY, A HALOGEN RETENTION VENT HEARS 3 Sheets-Sheet5 Original Filed Feb. 5, -1971 FIG 7b nited States Patent 3,827,915 IN ABATTERY, A HALOGEN RETENTION VENT MEANS Ralph Zito, Jr., West-ford,Mass., assignor to The Zito Company, Inc., Derry, N.H.

Original application Feb. 3, 1971, Ser. No. 112,254, now Patent No.3,719,526. Divided and this application Oct. 30, 1972, Ser. No. 302,260

llnt. Cl. H01m N06 US. Cl. 136-179 2 Claims ABSTRACT OF THE DISCLOSUREElectrode structures for a rechargeable metal halide battery include: acathodic electrode comprising a halogen-inert electroconductive layerand bonded to one of the major surfaces thereof, a halogen-entrapmentstructure comprising a halogen-adsorbent layer and a surface layercomprising porous, electrically non-conduct1ve halogen-inert,electrolyte-inert, and halogen non-adsorbent particles, having anaverage largest dimension less than about 10 mils and a halogen-inertbonding agent bonding the particles together into an integralnon-conductive mass which essentially retains the porosity of theparticles; and an anodic electrode having a coating of such electricallynonconductive particles on the electroplating surface of the electrode.

This is a division of application Ser. No. 112,254 filed Feb. 3, 1971,now US. Patent 3,719,526.

This invention relates to metal halide storage batteries and also tosimilar devices requiring halogen storage systems.

In a rechargeable metal halide storage battery, a metal halide salt,dissolved in a suitable electrolyte, is electrolyzed during chargingproviding free metal at the anode and molecular halogen at the cathode.If the battery is to retain substantial charge after the chargingcurrent is discontinued, the metal and halogen must be storedsubstantially out of chemical contact with one another, and yet still beaccessible on demand when power is to be drawn from the battery. Thismust be accomplished in a compact battery construction, with a smallelectrolyte volume, if the battery is to be an economical energy source.Moreover, to have an adequate useful life, the storage system providedmust function reliably and without deterioration over repeated chargingand discharging cycles.

The object of this invention is to provide compact, reliable andeconomical metal halide energy sources, such as rechargeable storagebatteries, having improved charge retention capacity.

Another object is to provide halogen storage systems for storing halogenin molecular form for substantial periods of time in a medium renderingthe halogen substantially instantaneously accessible on demand forelectrochemical react-ion.

A further object is to provide improved electroplating surfaces forreversibly storing electroplated metals in a medium rendering theelectroplated metal substantially instantaneously accessible on demandfor electrochemical reaction.

A particular object is to provide improved cathode and anode structuresfor metal halide batteries which are light weight, simple and economicalto produce in mass quantitles, and which interact with halogen/ halideand metal/ metal ions, respectively, in a consistent, reproduceablemanner during repeated charging and discharging operations.

Another object is to provide improved rechargeable zinc bromide storagebatteries.

The invention features electrode structure for a re chargeable metalhalide battery in which a salt of an electroplatable metal and a halogenselected from the class consisting of chlorine, bromine and iodine iselectrolyzed from solution in a liquid electrolyte medium during thecharging cycle and reformed during the discharging cycle. In one aspect,the invention features electrode structure having improved cathodecharacteristics comprising a halogeninert electroconductive layer,substantially impermeable to the electrolyte, and substantiallyimpermeable to the electrolyte, and substantially impermeable to halogenbetween discharging cycles, which layer has bonded to it along one ofits major surfaces, a halogenentrapment structure, which is permeable toelectrolyte, inert to halogen, and comprises a halogen-adsorbent layeradjacent the electroconductive layer, and, at least along and bonded tothe major surface of the adsorbent layer (opposite that bonded to theelectroconductive layer) a surface layer comp-rising porous,electrically non-conductive halogen-inert, electrolyte inert, andhalogen non-adsorbent particles, having an average largest dimensionless than about 10 mils and a halogen-inert bonding agent bonding theparticles together into an integral non-conductive mass whichessentially retains the porosity of the particles. When the battery ischarged, this surface layer retards halogen diffusion out of theadsorbent layer and into the electrolyte, where it could otherwisemigrate to and attack the metal electroplated at the anode surface ofthe battery. The charge storage capacity of the battery is substantiallyincreased over batteries utilizing adsorbent layers alone (ofsubstantially the same thickness). Yet, this added surface layer, beingnon-reactant with and nonadsorbent of halogen, will not adversely affectthe rate at which halogen can recombine with metal in the battery duringdischarge, thus permitting selected high discharge rates. The halogenwill not form at this surface layer in the charge cycle, since the layeris electrically non-conductive, but will form beneath the layer, andhence will not tend to be exposed to the electroplated anode surfaceswhen the battery is left at full charge. This surface layer is alsopreferably thin generally about A or less of the total thickness of theunderlying adsorbent layer), hence permitting close spacing ofelectrodes, minimal electrolyte volume, and resultant light-weight andcompact battery construction.

Where the halogen is bromine, a preferred adsorbent layer comprisesbromine-adsorbent activated carbon particles (preferably comprising ormore of the total weight of the layer) bonded together into an integraladsorbent mass by a bromine-inert bonding agent, the adsorbent layerhaving a bromine adsorptivity of at least 0.5 gms, of bromine per gramof adsorbent layer. In a preferred entrapment device having such anadsorbent layer, the surface layer bonded thereto has edge portions ofgreater thickness than its interior portion, and the edge portions ofthe adsorbent layer are of correspondingly reduced thicknesses, so thatthe exposed cathode surface of the surface layer may be flat. This edgeconstruction assures that the major adsorbent surfaces will be fullycovered by the surface layer, further improving the charge retentioncapacity of the battery. In such constructions, a preferred thicknessfor the thinner interior portion of the surface layer is about 10 to 30mils.

In another aspect, the invention features electrode structure, havingimproved anode characteristics, comprising a halogen-inertelectroconductive member substantially impermeable to said liquidmedium, substantially inert to halogen at least between dischargingcycles, and which has an exposed surface, adapted to be arranged as ametal electroplating surface, to which is firmly bonded a coating of theaforesaid non-conductive particles of a thickness sufficient tosubstantially cover the exposed surface. It has been found that such anelectroplating surface results in a more consistent open circuit voltageat successive charging cycles, indicating a smoother, more uniform, andstronger electroplate deposit. The discharge cycle is also found to besteady, smooth and predictable. It is believed that the electricalnon-conductivity of the coating causes metal to commence plating at theinterface of the electroconductive layer and the coating, and hence wellwithin the coating itself. Subsequent metal layer growth being withinand through this coating, including within the porous interiors of theparticles, the electroplated metal layer has strength and integrity.Further, since substantial portions of this electroplate are somewhatisolated from whatever free halogen may be in the electrolyte, corrosionor dissolution of the electroplate during storage of the battery in acharged condition is considerably impeded. The uniformity of theelectroplate deposit over the entire electroplating surface furtherdiscourages the growth of metal dendrites through the electrolyte which,upon reaching electrolytic cathode surfaces, can cause discharge in oreven shortcircuit of the battery. In preferred embodiments, this coatingis about to 20 mils thick.

In the preferred anodic and cathodic structures, the electroconductivelayer or member comprises highly electroconductive carbon particles(e.g., graphite) bonded together into an integral electroconductive massby a bonding agent substantially inert to bromine and to theelectrolyte, the mass being preferably 25% to 75% carbon particles byweight.

Other objects, features and advantages will appear to one skilled in theart from the following description of a preferred embodiment of theinvention taken together with the attached drawings thereof, in which:

FIG. 1 is a plan view of a metal halide battery embodying the presentinvention, with the forward side of the battery housing partially brokenaway;

FIG. 2 is a sectional view of a terminal cathode con-- structed inaccordance with the present invention;

FIG. 3 is a sectional view of a composite or intermediate electrodeconstructed in accordance with the present invention;

FIG. 4 is a sectional view of a terminal anode constructed in accordancewith the present invention (or used in the battery of FIG. 1);

FIG. 5 is a plan view, partially broken away, of the cathode of FIG. 2;

FIG. 6 is a plan view, partially broken away, of the terminal anode ofFIG. 4',

FIGS. 7a, 8a, 9a and 10a are plan views of electrode structures usefulin forming the electrode shown in FIG. 3; and

FIGS. 7b, 8b, 9b and 10b are sectional views of the electrode structuresof FIGS. 7a, 8a, 9a and 10a, respectively.

FIG. 1 shows a zinc bromide battery 10 encased in a polyethylene housing12, of which the forward side has been partially broken away to exposethe interior of the battery. The battery has a terminal cathode 14electrically connected to a cathode terminal screw 15 which extendsthrough the housing 12, a terminal anode 18 at its other end which iselectrically connected to an anode terminal screw 20 which also extendsthrough the housing 12, and a plurality (7) of intermediate or compositeelectrodes 22, all arranged generally in parallel and spaced apart fromone another. A liquid electrolyte medium 23 consists of an aqueoussolution which is 0.5 to 7 molar zinc bro mide, .01 molar aluminumchloride, and .005 molar aluminum potassium aluminum sulfate (thealuminum salts being brightener additives). This battery is about 14volts, and has a capacity of over 20 amp/hour at full charge.

Referring to FIGS. 2 and 5, the terminal cathode 14 has a copper screen26, the major portion of which is sandwiched between two bromine-inertand bromine-impermeable electro-conductive layers 28, 29, each of whichis formed of a 50-50 by weight mixture of electroconductive graphiteparticles bonded together by poly (vinylidene fluoride) (Kynar)particles, the graphite and bonding agent being bonded together underheat and pressure to form an integral electroconductive layer. Eachlayer 28, 29 is capable of conducting electricity across its thicknessfrom its exposed face through copper screen 26 and has a thickness ofabout 25 mils. The layers 28, 29 are bonded to one another through theopenings in copper screen 26.

Firmly secured to the exposed face 30 of layer 29, which has an exposedsurface area of about 120 (10 in. x 12 in.) square inches is a bromineentrapment structure 32, which consists of an interior bromine adsorbentlayer 34 and a surface layer 38 firmly secured to the front exposed face36 of adsorbent layer 34. Adsorbent layer 34 is formed of at least aboutby weight of bromine adsorbent activated carbon particles, and theremainder of a polyethylene bonding agent effective to bond the carbonparticles into an integral adsorbent mass. The layer 34 has a thicknesson the order of about mils over most of its area, and a somewhat reducedthickness at its edges 37.

Surface layer 38 is about 30 mils thick over most of its area, and ofsomewhat greater thickness at its edges 42, and is formed of at leastabout 90% by weight of electrically non-conductive particles bondedtogether into an integral non-conductive mass by a polyethylene bondingagent. The particles are porous, inert to halogen and electrolyte, anddo not adsorb halogen. The edges 37 of adsorbent layer 34 are roundedaround the entire periphery of the layer, and the edges 42 of thesurface layer 38 are sized so that the bromine entrapment device 32 willhave a rectangular cross section in all three perpendicular planes,presenting a fiat extended cathode surface 44 to the electrolyte.

Each composite electrode 22, shown in FIG. 3, has a singleelectroconductive layer 46, identical to either of layers 28 and 29 ofterminal cathode 14, and approximately 25 mils thick. A thin coating 50(on the order of 10 mils) of the above-described electricallynon-conductive particles is bonded to the exposed anode surface 48 ofplastic layer 46. These particles are bonded directly to the poly(vinylidene fluoride) bonding agent of electroconductive layer 46.Secured to the opposite surface 52 of electroconductive layer 46 is abromine entrapment structure 54 identical to the bromine entrapmentstructure 32 of terminal cathode 14.

Referring to FIGS. 4 and 6, terminal anode 18 has a. copper screen 56,identical to copper screen 26 of terminal cathode 14, the major portionof which is sandwiched between two bromine inert and bromine impermeableelectroconductive layers 58, 59, which are identical to theelectroconductive layers 28, 29 of terminal cathode 14 and areidentically bonded to one another through the openings of screen 56. Theexposed anode face 62 of electroconductive layer 58 has a thin coating64 of the above-described electrically non-conductive particlesidentical to the coating 50 on the anode face 48 of composite electrode22. Exposed portions of the electroconductive layers, and bromineadsorbent layers are coated with a thin coating 66 of a bromine-inertand gas-permeable sealer, such as the silicon rubber sealer availablefrom General Electric Company under the trade name RTV.

Although the battery shown in FIG. 1 is electrically tapped as shownonly at the terminal electrodes 14 and 16, it will be understood thatlesser voltage may be tapped by providing intermediate terminal-typeelectrodes. Such electrodes would be identical to terminal cathode 14,except for the presence of a thin coating of electrically nonconductiveparticles on the exposed major surface of the electroconductive layer28, identical to the coatings 50, 64 on composite electrode 22 andterminal anode 18, re spectively.

The battery housing 12 has in its cover 80 an opening 81, sized toreceive a generally cylindrical gas escape cap 82 which seals theinterior of the battery. As shown in FIG. 1, cap 82 is formed of agas-permeable (but liquid and solid-impermeable) and bromine-inertmaterial, such as polyethylene. The cap is at least partially filledwith a material which will remove bromine from gases coming from thebattery through the interior cap wall, but not other gases, such asoxygen and hydrogen. Finely-divided zinc filings is a suitable suchmaterial, as is particulate activated carbon. The vent cap 82 is sizedto fit in a tight seal into opening 81 and has a flange 88 which withsealing gasket 89, provides a gas tight seal between the cap 82 and thecover 80. Since the gas escape cap is gaspermeable, but at least itsinterior wall is liquid-impermeable, hydrogen and oxygen gases evolvedin small quantities during operation of the battery (but not bromine orliquid) will escape through the exterior cap wall to the atmosphere.

The bottom wall 91 of housing 12 has a number of upstanding ribs 92,defining therebetween grooves 94. A spacer 95 has the electroductivelayers protruding therethrough, and sealed therein by a bromine-inert,and bromine, and liquid-impermeable epoxy resin 96. The spacer 95 has anelongated opening 97 (about 1 inch wide) for filling the battery.Screens 26, 56 and screws 15, 20 are also potted, as shown, in resin 96.

When the electrodes are secured in place, the battery is filled withelectrolyte solution by vacuum impregnation, so that thebromine-adsorbent device becomes saturated with electrolyte all the wayin to the electroconductive (graphite-fluorocarbon) layers. The batteryis charged in the usual manner across the terminal screws 15, 20,electrolyzing the zinc bromide salt to form molecular bromine which isadsorbed by the activated carbon of the cathode and metallic zinc, whichis electroplated onto the anode surfaces. During the discharge cycle,the opposite electrochemical reaction takes place, the molecular brominereturning to bromide ion and the zinc plating dissolving to zinc ions.

Although the electrochemical system illustrated is the zinc bromidesystem, the illustrated electrode structures may be suitable for usewith other metal halide systems, where the halogen is chlorine oriodine, or the metal is other than zinc. Among the other metals whichare reversibly electro-platable, and form water-soluble metal halidesalts are nickel, cadmium, tin, lead and copper. In a non-aqueouselectrolyte medium, such as might be utilized for chlorine, the list ofmetals might also include sodium, potassium and lithium. Of thesesystems, the zinc bromide system has the advantages of providing areasonably high potential (1.83 volts), using a very soluble salt whichprovides a low resistivity electrolyte, and having a calculated freeenergy per pound of about 200 watt/hours. The molarity of a zinc bromideelectrolyte during char-ging and discharging is preferably between about0.5 and 7. The electrolyte also may contain a brightener to improve zincelectroplating.

Among preferred particles for forming the cathode surface layers andanode coatings are those commonly used as filter materials or filteraids such as diatomaceous earth, molecular sieves, zeolites and thelike. These particles are characterized by a high water absorptivity,typically more than two times their weight, and an electricalresistivity on the order of ohm-cm. or even greater.

A particularly preferred material is a flux-calcined diatomaceous earthavailable from Johns-Manville Company under the trade name Celite 560.

The pore size of the particles must be large enough to permit diffusionof electrolyte therethrough but small enough to at least substantiallyretard diifusion of molecular bromine. Preferred particle layers arecomposed of particles, the pores of which have an average effectivediameter less than about one micron, and preferably lower, down tomolecular size."

The size of the non-conductive particles is chosen to provide surfacelayers at least several particles thick. It is preferred thatsubstantially all of the particles have a largest dimension less than 10microns. Particles which pass a 100 mesh screen are preferred (theaforesaid Celite 560 particles so passing, but being 6070 retained on a150 mesh screen). Particles in the micron range and below are alsouseful, so long as the particles are not so small as to be occluded bythe bonding agent used. It is preferred that the absorptivity of thesurface layer be essentially that of the particles themselves, with theamount of bonding agent being that barely sufiicient to provide anintegral layer. Preferably, the surface layer is at least about byweight of the particles, and no more than about 10% by weight of bondingagent- The bonding agent in the surface layer must be adherable to thatin the adsorbent layer, and, preferably, is identical thereto. Usefulbonding agents include polyfluorocarbons, such aspolytetrafluoroethylene (Teflon, available from E. I. du Pont de Nemours& Co.), poly (vinylidene fluoride) (Kynar, available from Penwalt Co.),polymonochlorotrifluoroethylene (CTFE, available from Allied ChemicalCo.), and FEP, a fiuorinated polyethylene available from the same DuPont; poly (vinyl chloride) homopolymers (plasticized or unplasticized)(e.g., Geon 222, available from B. F. Goodrich Co.); poly (vinylidenechloride) homopolymers and copolymers (50% or greater vinylidenechloride) such as acrylonitrile and vinyl chloride copolymers (availablegenerally under the trade name Saran from Dow Chemical Co.);polymethacrylates such as poly (methyl methacrylates) (Plexiglas,available from Rohm & Haas Co.); and polyalkylenes such as polyethyleneand polypropylene. The polyalkylenes are presently preferred for theadsorbent layer-because of their low working temperatures, inertness tobromine, and availability at low cost.

The anode coating may be simply electrically non-conductive particles asdescribed pressed into and bound to the preformed electroconductivemember.

A thicker layer, having a thickness on the order of that of the surfacelayer of the cathode is also useful. For forming such a layer, apolyalkylene bonding agent is preferred, and the mixture of polyalkyleneis bonded to the already pressed and bonded particle coating, in thesame manner as hereinafter described for adhering the adsorbent layer toa base electroconductive layer with reference to FIGS. 8a, 8b, 9a, and9b.

In addition to the illustrated poly (vinylidene fluoride) bonding agentfor the electroconductive layers of the composite electrodes 22, and theillustrated polyethylene bonding agent in the various adsorbent layers,other bonding agents or mixtures of bonding agents may be substitutedtherefor.

Materials suitable for forming such layers are set forth in theapplicants copending US. patent applications: Ser. No. 867,799, filedOct. 20, 1969, entitled Battery; and, Ser. No. 872,993, filed Oct. 31,1969, entitled Metal Halide Battery to which reference is made fordetails of selection of bonding agents. Another useful bonding agentsystem is the mixture of a polyfluorocarbon with a minor amount of apolyethylene or polypropylene, such as disclosed in the assigneescopending US. patent application, entitled ElectroconductiveComposition, Ser. No. 109,156, filed Jan. 25, 1971, now abandoned, inthe names of Ralph Zito, Jr., and Edward M. Russell. Where such amixture is used, bonding of the polyfiuorocarbon-bondedelectroconductive layers to the polyethylene-bonded activated carbonlayer is simpler. For example, it is possible, but not necessarilypreferable, to eliminate the step of adhering a thin activated carbonlayer, as shown in FIGS. 8a, 8b, to the electroconductive layer prior toforming the bromine-adsorbent device.

Where the electroconductive layer is to be part of a terminal anode orterminal cathode having a brominecorrodible screen, the bonding agentmay be any of those previously set forth for the composite electrodes,except the bromine-permeable polyethylene and polypropylene. Wherebromine-degradable polymers, such as vinyl chloride and vinylidenechloride are employed, in electroconductive layers, whether in compositeor terminal electrodes, the maximum bromine concentration at theelectroconductive layers should not exceed about 0.5 M.

Whether the composition of the various electrodes, each electrode shouldhave a total interface resistance, per square inch of thecross-sectional electrolyte-contacting surface area, not greater thanabout 0.05 ohms. Inaddition, each electroconductive layer 28, 29, 46,58, 59 should have a volume resistivity, such that d is not greater thanabout 0.1 ohm-in where d is the thickness of the electroconductivelayer.

For terminal electrodes, the electroconductive layer which is notexposed to the remainder of the battery (the layer 28, for example, ofterminal cathode 14, need not contain any electroconductive carbonparticles, but may be simply a sheet of the identical bonding agentsused in the electroconductive layer to which it is secured through thecopper screen.

FIGS. 710 show a method for making the electrodes 14, 18, 22, the methodfor forming the composite electrode 22 being used as a particularillustration. The electroconductive layer 46 shown in FIGS. 7a and 7b isformed by mixing together 50 grams of poly (vinylidene fluoride)(Pennwalt Kynar 301) with 50 grams of highly electroconductive graphiteparticles (Dixon No. 1112). This mixture is introduced into a 10 inch X12 inch frame and trowelled until level. The frame is placed in a moldformed of two platens, which are then heated to 450 for one minutewithout pressure, and then are pressed at 500 p.s.i. and 450 F. forthree more minutes. The frame is then transferred to platens at roomtemperature and cooled between those platens under 500 p.s.i. for twomore minutes. The resultant layer is about 25 mils thick. Where aterminal anode 18 or a terminal cathode 14 is to be made, two suchlayers 46 are provided. A copper screen is placed between the twoelectroconductive layers, with a portion of the screen protruding formaking ultimate electrical connection thereto, and the layers and screenare placed between platens, and heated for one minute without pressureat 450 F. and for two more minutes at 500 p.s.i. and 450 'F.,transferred to platens at room temperature, and cooled for two minutesbetween these platens at 500 p.s.i.

Referring to FIGS. 8a and 8b, there is spread in the bottom of a frameas described above a thin layer (about 10 mils) of Celite 560, a fluxcalcined diatomaceous earth available from Johns Manville Company whichhas a dry density of 19.5 pounds per cubic foot, a particle size suchthat all of the particles pass a 100 mesh screen and 60% of theparticles by weight are retained by a 150 mesh screen and a waterabsorption of about 220% by weight. These particles have pores witheffective diameters less than 1 micron. The electroconductive layer 46is placed in the frame on top of the Celite layer, and there is spreadonto the other exposed major surface of the electroconductive layer 46 athin layer (about 2 grams), confined within a frame enclosing a 9 in. x9 in. area, of activated carbon particles (Barneby Cheney UU Grade). The9 in. x 9 in. frame is removed, and the Celiteelectroconductivelayer-activated carbon is placed between two platens, heated to 375 F.for one minute without pressure, compressed between the platens at 375F. and 400 p.s.i. for two minutes, transferred to platens at roomtemperature and cooled for two minutes between these platens at 400p.s.i. Both surfaces of the resultant sheet are then scrubbed with aconventional stiff scrubbing brush to remove non-bonded activated carbonand Celite particles. The 375 temperature having been sufficient tosoften slightly the poly (vinylidene fluoride) binder of theelectroconductive layer without affecting the basic shape and form ofthe layer, the Celite and activated carbon particles are bonded toopposite sides of the layer by the poly (vinylidene fluoride) bondingagent, forming the integral electrode structure shown in FIGS. 8a and8b.

Onto the activated carbon surface of this electroconductive structure istrowelled, within the 9 in. x 9 in. frame, a mixture consisting of 72grams by weight of the aforesaid activated carbon and about 3.6 grams byweight of polyethylene particles (EN-510 powder, available from US.Indus. Chem. Co.), this mixture having been previously ball milled forfourteen minutes. The mixture is trowelled in such a manner as to leaverounded surfaces around the edges of the activated carbon layer, so thatthe top surface has an area of about 8% in. x 8% in., thecross-sectional area of the activated carbon gradually increases toabout 9 in. x 9 in. about half way down the total thickness of the layer(about 60 mils down from the top surface). This electrode structure isshown in FIGS. 9a and 9b.

There is trowelled onto this activated carbon-polyethylene mixturewithin the 9 in. x 9 in. frame a mixture consisting of 32.4 grams of theaforesaid Celite 560 and 3.6 grams of the aforesaid polyethylene. TheCelite mixture is trowelled in such a manner as to fill the edgeportions of the activated carbon layer of FIGS. 9a, 9b, to provide, asshown in FIGS. 10a, 10b, a composite activated carbon Celite layer ofrectangular cross-section. The entire composite of FIGS. 10a, 10b isplaced between plantens, heated to 300 F. for 7 minutes, pressed at 75p.s.i. and 300 F. for 7 minutes, transferred to platens at roomtemperature, and cooled at 75 p.s.i. for 7 minutes. The resultingsurface (Celite) layer retains essentially the porosity of theindividual particles. The interior portion (exclusive of the edges) ofthe activated carbon is about mils thick, and the interior portion(exclusive of the edges) of the Celite layer is about 30 mils thick.

Other embodiments will occur to one skilled in the art and are withinthe following claims.

What is claimed is: 1. In a rechargeable metal halide battery in which asalt of an electroplatable metal and a halogen selected from the classconsisting of chlorine, bromine and iodine is electrolyzed from solutionin a liquid electrolyte medium during the charging cycle and reformedduring the discharging cycle, gas escape means comprising aliquid-impermeable, gas-permeable first barrier arranged to seal theinterior of said battery,

a gas-permeable second barrier spaced from said first barrier andarranged exterior of said first barrier,

and a halogen-retention substance disposed between said barriers forretaining halogen passing from the interior of said battery through saidfirst barrier.

2. The battery of claim 1 wherein said halogen retention substancecomprises zinc.

References Cited UNITED STATES PATENTS 3,682,703 8/1972 Smith 1361551,505,939 8/1924 Batchelor 136-179 2,615,062 10/1952 Craig 1361793,285,781 11/1966 Zito 136-6 R ALLEN B. CURTIS, Primary Examiner H. A.FEELEY, Assistant Examiner

